The present invention generally pertains to building mix beds of variable bulk materials, from which slices of the bulk material are reclaimed from time to time, and is particularly directed to improved systems, methods and computer readable storage media for determining the compositions of the mix beds as they are being built and for the stacking of the bulk materials onto the mix bed in such a manner as to build a mix bed having a desired composition throughout the mix bed. Variable bulk materials include different types of bulk materials and/or bulk materials of a given type having variable compositions.
Mix beds, also known as blending beds, are widely used in the cement industry and other industries, such as coal processing, mining, grain and fertilizer, to reduce variability in the bulk materials when processed, as well as to provide a stockpile for processing when the material production or material delivery systems are halted. Although mix beds are sometimes used to homogenize material delivered from a single source, mix beds frequently are used to homogenize different materials respectively delivered from different sources, such as mixtures of limestone and clay for use by cement plants and mixtures of high and low sulfur coal for use by power plants. It is an object of the present invention to reduce variability throughout the mix bed to such an extent that further homogenization will not be required.
Mix beds are composed of many layers of bulk material. FIG. 1 shows a typical longitudinal mix bed 10 in which a stacker 12 moves back and forth along the ridge 14 of the mix bed 10 and stacks many layers of variable bulk materials. The bulk material is delivered to the stacker 12 from a selected source of bulk materials by a conveyor 15. FIG. 2 is a vertical sectional view of the mix bed 10 taken normal to the movement of the stacker 12. The bulk material stacked on the ridge 14 of the mix bed 10 by the stacker 12 tumbles down the faces of the mix bed 10 to thereby distribute itself in chevrons, as shown in FIG. 2; whereby the respective layers L become thinner as the height of the mix bed 10 increases, as also shown in FIG. 2. While one mix bed 10 is being built, slices of bulk materials are being reclaimed by a reclaimer 16 from a previously built mix bed 10'. The reclaimer 16 removes the bulk material from the mix bed 10' while advancing from one end of the mix bed 10' to the other. Reclaimers 16 typically reclaim bulk material from a mix bed 10 at an angle with respect to the stacked layers L that is inclined to be equal to or slightly larger than the angle of repose of the bulk material. By this process, material from a large number of layers L is simultaneously recovered from the mix bed 10' and thereby blended to be of an aggregate composition combining the respective compositions of the blended layers L. Normally, the respective compositions of the layers are selected to produce the desired aggregate composition of the entire mix bed 10. Since the layers are independent from one another, variations are mainly random, and the reclaiming process recovers a portion of a large number of layers L simultaneously, with the homogenization effect being approximately proportional to the square root of the number of layers L intersected by the reclaimer.
FIG. 3 shows a continuous-mode circular mix bed 20, that is built by a rotating stacker 22 continuously stacking bulk material onto a ridge 23 that is thereby continuously extended toward a continuously extending blending tail 24 at one end of the mix bed 20 while bulk material is being continuously reclaimed from the other end 25 of the mix bed 20 by a rotating reclaimer 26; whereby there is no need to move the stacker 22 and the reclaimer 26 from one mix bed 20 to another. Both the stacker 22 and the reclaimer 26 rotate in the same circular direction 27 about a common central axis 28. The bulk material is delivered to the stacker 22 from a selected source of bulk material by a conveyor 29. The bulk material stacked onto the ridge 23 by the stacker 22 tumbles down the faces of the mix bed 20 to thereby distribute itself in layers, as shown in FIG. 2. The stacker 22 moves back and forth in the circular direction 27 along the ridge 23, typically at a fixed elevation above the ridge line, to make overlapping layers, as shown in FIG. 4, which is a vertical sectional view of the mix bed 20 taken in alignment with the circular direction of movement 27 of the stacker 22. The respective layers L are shown to be of an arbitrary constant size but are physically distinguishable from each other only where there is a variation in the bulk material being stacked onto the ridge 23. In the prior art, the length of the blending tail 24 is determined by the homogenization requirements for the bulk material. As shown in FIG. 4 the layers L of the circular-mode mix bed 20 are stacked in an inclined fashion with the angle of the incline chosen to yield the desired tail length but being equal to or less than the angle of repose of the bulk material. These layers L have a length determined by the incline angle and the finished height of the ridge 23. The reclaimer 26 reclaims the bulk material from the mix bed 20 in the same manner as the reclaimer 16 reclaims bulk from the mix bed 10', as described above with reference to FIG. 1.
In order to build a mix bed having a desired aggregate composition, predetermined quantities of selected bulk materials are delivered to the mix bed for stacking by the stacker in accordance with a schedule so that the respective slices of bulk material that are reclaimed from the mix bed in accordance with the homogenizing capability of the mix bed are of a desired aggregate composition within an acceptable range of aggregate compositions. Sometimes, however, due to equipment failure or other problems, such as variations in the composition of a particular selected bulk material, one or more of the selected bulk materials are not immediately available, thereby making it impossible to build the mix bed in accordance with a schedule. If the mix bed building process is not interrupted when this condition occurs, it is possible for various sectors S of the mix bed to have an undesired aggregate composition. A sector is a three-dimensional lateral segment of arbitrary size and extending between the top and the bottom and toward the sides of the mix bed 10, 20 in an orientation that is vertical, as shown in FIG. 4, and approximately normal to the direction of advancement of the reclaimer 16, 26. In a circular mix bed 20, a sector S may also be delineated by radii and a portion of the mix bed circumference.
Various on-line analyzers and/or sampling techniques are used to determine the aggregate composition of a mix bed as the mix bed is being built so that an undesired aggregate composition of a given sector S of the mix bed can be recognized and then corrected. For example a bulk material analyzer, such as an analyzer utilizing prompt gamma-ray neutron activation analysis (PGNAA), is disposed about a conveyer delivering bulk material to the stacker to analyze the composition of the bulk material being delivered to the stacker; and the quantity of the bulk material of such composition that is being delivered to the stacker is measured gravimetrically by means such as a scales disposed within the conveyor assembly.
A disadvantage incident to building continuous-mode circular mix beds 20 when compared to building longitudinal mix beds 10 has been an inability to correct an undesired aggregate composition of an individual sector S located between the top of the blending tail 24 and the other end 25 of the circular-mode mix bed 20 without having to interrupt the continuous building of the mix bed 20. Longitudinal mix beds 10 are considered to be of a consistent aggregate composition between, but not at, the end portions because they are constructed from hundreds of thin horizontal layers covering the full length of the mix bed. An undesired aggregate composition of a longitudinal mix bed 10 can be adjusted at any time simply by adding additional layers of a corrective bulk material; whereas bulk material is added to a continuous-mode circular mix bed 20 only on the blending tail 24 such that those sectors S of an undesired aggregate composition located between the top of the blending tail 24 and the other end 25 of a continuous-mode mix bed 20 where the bulk material is being reclaimed can no longer be corrected during the continuous mode of operation and can be corrected only by adding corrective material to those sectors after the continuous building of the mix bed 20 has been interrupted. For this reason many operators of circular mix beds have chosen to operate them in a batch mode that permits correction in the same manner as in a longitudinal mix bed, wherein corrective bulk material is added uniformly over a batch that caused the mix bed to be of an undesired aggregate composition.
One prior art technique for correcting aggregate-composition irregularities detected in a continuous-mode circular mix bed while building the mix bed, as described by Ahrens, "Latest Developments in Circular Mix Bed Technology", Bulk Solids Handling, Vol. 17, No. 2, April/June 1997, pp. 257-263, is to sample and analyze the composition of the different layers of the circular mix bed as the circular mix bed is being built and to stack an overlapping layer of corrective bulk material when it is determined from such analysis that the layers of bulk material being overlapped are not of a predetermined aggregate composition. However, such technique does not appear to be applicable for correcting aggregate-composition irregularities detected in given sectors S of the mix bed 20 while building the mix bed 20.